Several lamps have been developed in an attempt to provide uniform illumination of a relatively large work area such as that found in an operating theatre environment. Examples of such lamps are disclosed in U.S. Pat. Nos. 4,159,511 to Dejonc (using concave reflecting surface), 4,153,929 to Laudenschlarger (using multi-faceted reflector), 4,135,231 to Fisher (using coaxially-arranged, curved reflectors with a single movable light source), 3,732,417 to Nordquist (using a conventional circular reflector and prismatic lens system), 3,360,640 to Seitz et al. (using multiple, individual light sources from individual fiberglass light-conducting bundle), 3,225,184 to Reiber (using several individual light fixtures directed onto a field), 2,827,554 to Gunther et al. (using several individual light fixtures directed on a field), and 2,495,320 to Franck (using a plurality of individual light sources, each having a horizontal square 2-component refractor). All of the above references are hereby incorporated into the present disclosure by reference. Lamps of the above types have been unsatisfactory because they have failed to provide both a desired degree of uniform luminosity together with a sufficient depth of field such that the light source may be conveniently moved about the task surface without adversely affecting the luminosity characteristics.
The present invention represents an improvement in a type of lamp different from the above-referenced systems An early lamp of this type was described by Blin in French Patent No. 1,495,007. The Blin reference teaches a lamp whose light source resides above a field of concentrically circular prisms. The light emanating from the light source passes through a toroidal lens (such as a Fresnel-type lens) which renders the beam substantially columnar and directs the columnar beam onto the prism field where the columnar beams are redirected (by internal reflection) and concentrated (by action of the prism curvature) onto a target field below the prism plane.
More recently, U.S. Pat. No. 3,941,993 to Hubert improved upon the Blin lamp by the use of straight prisms across radial sectors of the planar prism field so as to produce a prism field resembling a spider web design. Such construction provided columnar light beams emanating from a toroidal lens system so as to impinge upon straight prisms which maintain the columnarity of the light beams and by reflection overlap them from all radial sectors of the prism field into the illuminated target field. The result was an illuminated target field of greater width without the greatly intensified illumination ("hot spots") which resulted from the concentrically, inwardly curved prisms of Blin.
One of the remaining problems associated with the lamp proposed by Hubert is that its resultant, intersecting, columnar light beams provide only a narrow depth of field with uniform luminosity. The region of best illumination in such a lamp occurs over a working distance (i.e., distance from the lamp to the target field) which is quite small, and which is achieved at a point where all of the patches of light from each prism sector overlap. This effect is illustrated by the luminosity curves shown in FIG. 1A. The luminosity curve of the Hubert lamp is represented by the broken lines in FIG. 1A while the luminosity curve of the present invention is represented by solid lines. FIG. 1A shows that a lamp in accordance with the Hubert reference achieves uniform luminosity (seen as a plateau in the luminosity curve) at distances from about 40 inches to about 54 inches from the prism plane. At distances greater than 54 inches from the prism plane, it can be seen in FIG. 1A that the luminosity curve of the Hubert lamp becomes depressed in the center of the field as the intersecting columnar beams begin to diverge from their point of intersection. In practice, this effect manifests itself as a doughnut-shaped illumination with a dark center.
It has heretofore not been recognized that the depth of field can be improved by controlling the divergence, in the angular direction, of the light leaving each of the individual prisms from a given radial sector.
It is therefore desirable to produce a lamp which achieves both uniform luminosity (i.e., through the intersection of non-focused light beams) and which provides a much greater depth of field and makes such uniform illumination available to the user over a much greater range of distances between the lamp and the object or surface to be illuminated. Uniform luminosity is particularly critical in an operating theatre environment because the task surface is generally three-dimensional and particularly prone to shadowing. Providing uniform light from a number of radial sectors helps eliminate such shadowing. Greater depth of field is also important due to the desirability of having the lamp movable so as to illuminate from varying distances. This allow clearance for equipment and members of the surgical team.
It has also been the practice in the past to support such fields of prisms upon a transparent plate. A disadvantage of such an arrangement is that if such a plate becomes soiled or scratched, it must be cleaned and/or replaced. Removal of the plate for either purpose causes all of the prisms to be displaced from their normal positions in the lamp.
Therefore, it is desirable to produce a lamp of the above-described type whose prism sectors are both self-supporting and protected from soil or damage from the lamp's outside environment.
According to the present invention it has been found advantageous to provide the toroidal lens system with a conical Fresnel lens made of plastic. Furthermore, the infrared filter is preferably made in two parts: a lower part treated to filter infrared rays and an upper non-filtering part so that none of the rays pass through the filtering part more than once. The protective transparent plate beneath the planar circular prism field is removable for cleaning or replacement without disturbing the prism field. The lamp handle, which depends from the lamp and is centered with respect to the prism field is vented to allow air to circulate about the light source to cool the light source. The light source itself, is mounted on a light source holding assembly. The light source holding assembly is preferably disposable and is replaced when the light source is replaced.